Conventional tools and overbalanced drilling methods may not be economically viable for drilling some hydrocarbon reserves having a narrow margin between the formation fracture and pore pressures, which would require excessive casing programs and larger, more expensive rigs. Underbalanced drilling, however, offers higher rates of penetration and reduces lost circulation as well as reducing formation damage. In some situations, underbalanced drilling can reduce problems associated with invasion of particulate matter into a reservoir, adverse clay reactions, phase trapping, precipitation and emulsification, and other issues. Underbalanced drilling can also be useful for mature reservoirs that have lower field pressure that makes the reservoir more susceptible to damage caused by traditional overbalanced drilling and completion technologies. Other advantages are also known in the art.
In underbalanced drilling, the pressure in the wellbore is purposefully maintained below the fluid pressure of the formation being drilled. Therefore, underbalanced drilling can use a lower density mud in formations having high pressures. As a more common alternative, inert gas such as nitrogen is injected into the drilling mud for the underbalanced drilling. During operation, a rotating control head at the surface allows the drill string to continue rotating and acts as a seal so produced fluids can be diverted to a separator. Any surface flow-rate measurements if made are typically measured after separator equipment using an orifice plate meter, which presents problems with measurement error and produce discrepancies between the wellhead flow rate and what is actually measured.
Keeping the drilling pressure under the reservoir pressure during underbalanced drilling requires the reservoir pressure to be either deduced or know during the drilling operation. Unfortunately, operators are not capable of knowing the formation pressure during real time, and the operators must estimate the formation pressure based on known pressures of other wells. For new wells in unproduced fields, for example, the pressure is likely to follow a hydrostatic gradient that can be determined from wireline formation tester surveys made in other appraisal wells. In this situation, the formation pressure is known when the first development wells are drilled, and pressure and rate transient analysis can identify a permeability distribution when the pressure is known independently.
When a new well is drilled into a depleted field, however, the major zones are likely to be at different pressures. The underbalanced drilling operation for such new wells is preferably designed using known pressures so that the appropriate degree of underbalance can be maintained during the operation. This pressure information can be obtained from nearby wells in which formation tester measurements have been made using either wireline or drillpipe techniques. For example, a borehole can be drilled in overbalance conditions, and a formation tester can be run in the borehole to obtain pressure information. For the underbalance operation, a sidetrack can then be drilled into the reservoir with minimal formation damage. If the formation pressure is not known accurately enough, then both permeability and pressure distributions need to be determined to characterize the reservoir. Designing an underbalanced drilling system and obtaining and using data from the underbalanced drilling operation to characterize a reservoir present a number of challenges. The subject matter of the present disclosure is directed to overcoming, or at least reducing, effects of at least some of these challenges.